The conflict has has killed more than 250,000 people and driven some 11 million from their homes

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This photo provided by the Syrian anti-government activist group Aleppo Media Center (AMC), shows a Syrian man carrying a girl away from the rubble of a destroyed building after barrel bombs were dropped on the Bab al-Nairab neighborhood in Aleppo, Syria, Saturday, Aug. 27, 2016.

A cease-fire came into effect in Syria at sunset Monday in the latest attempt led by the United States and Russia to bring some quiet in the 5 1/2-year civil war.

Residents and observers reported quiet in most of the country hours after the truce came into effect, though activists said airstrikes took place on contested areas around the northern city of Aleppo.

But the most powerful rebel groups have shown deep misgivings over the cease-fire deal, which was crafted without their input last weekend in Geneva between the top U.S. and Russian diplomats. Hours after it came into force, a coalition of rebel factions put out a statement that stopped short of committing to the cease-fire, a reflection of their distrust of the government.

The first week of the truce will be crucial. During that time, all fighting between the military of President Bashar Assad and rebels is to stop. But, Assad's forces can continue air strikes against the Islamic State group and al-Qaida-linked insurgents from the group once known as the Nusra Front.

However, the al-Qaida linked insurgents are closely allied to many rebel factions and are a powerful force in the defense of Aleppo in particular. That raises the danger that continued airstrikes will draw rebels into retaliation, eventually leading to the cease-fire's collapse, much as previous attempts earlier this year fell apart.

Compounding the situation, a group of 21 rebel factions issued a statement Friday in which they warned against targeting al-Qaida-linked militants. The statement was non-committal about whether the groups would abide by the cease-fire.

After a week, however, the conflict would potentially enter a dramatically different stage. A new U.S.-Russia coalition will step in to target former Nusra Front militants, and Assad's forces will no longer be permitted to. That will effectively remove Assad's pretext for war on opposition areas, which he calls a war on terror. Government forces will be allowed to fight defensively, target the Islamic State group and, in some designated areas, go after Nusra forces.

The deal's architects hope that would pave the way for an extended period of restraint that can serve as the foundation for peace talks between the war's many sides.

As the cease-fire came into effect, U.S. Secretary of State John Kerry said Monday that rebel factions must distance themselves from the al-Qaida-linked militants, whose group recently changed its name from Nusra to Jabhat Fatah al-Sham, or Levant Conquest Front.

He also said the Syrian government must allow deliveries of humanitarian aid into besieged areas, including the rebel-held districts of Aleppo.

Russia's Deputy Foreign Minister Mikhail Bogdanov said peace talks between opposition groups and the government could resume as early as next month.

Multiple rounds earlier this year in Geneva failed to make progress. Ultimately, talks have run into the question that neither side is willing to budge on — the fate of Assad and his government. As a result, the war has continued the grinding violence that has so far killed more than 250,000 people and driven some 11 million people, half of Syria's population, from their homes since 2011.

"It's an existential question for the regime as it currently stands. It's about the regime or not," said Lund.

In a letter to rebels disseminated last weekend, U.S. Special Envoy for Syria Michael Ratney promised them that, "our priority remains calming the situation to allow the launch of a credible political operation that leads to a true political transition that Syrians want most determinedly, a new Syria without Bashar Assad." A copy of the letter was given to The Associated Press by an opposition official.

Earlier Monday, a main opposition group linked to several small, moderate rebel factions said they will deal "positively" with the truce brokered by the U.S. and Russia. The Syrian National Coalition said that any effort that aims to end the suffering of the people "is a step in the right direction and we will deal with it positively."

Still, other rebel factions showed deep uncertainty. Some have complained that the cease-fire deal does not mention Assad's future and keeps in place the government siege of rebel-held parts of Aleppo.

"There is no balance in the agreement," said Col. Ahmad Hamada, an army defector who is now with the rebel group known as the Northern Division.

When the cease-fire went into effect at 7 p.m. (1600 GMT), the Syrian army issued a statement saying it would abide by a cease-fire until Sunday at midnight, while maintaining its right to defend itself against any violations.

Hours before the cease-fire went into effect, Assad vowed that his government would take back land from "terrorists" and rebuild the country. Assad spoke during a rare public appearance that included attending prayers for the Muslim holiday of Eid al-Adha in the Damascus suburb of Daraya, where rebels surrendered last month after a four-year siege.

The Britain-based Syrian Observatory for Human Rights, which has a network of activists around Syria to monitor the conflict, said "calm is prevailing on most of Syria's territories."

Still, residents in Aleppo reported some airstrikes and shelling, including a barrel bomb attack by government helicopters. It was not immediately known if the targets where Fatah al-Sham or other factions.

One of the more immediate goals of the U.S.-Russian agreement is to allow the U.N. to establish aid corridors into Aleppo, the contested northern Syrian city. Over 2,000 people have been killed in fighting over the past 40 days in the city, including 700 civilians and 160 children, according to a Syrian human rights group.